In a wireless communications network, when a UE moves away from a first cell and into a second cell, handover of the UE from the serving node of the first cell to the serving node of the second cell is necessary. A handover decision is generally made by the serving node of a UE based on measurement results reported by the UE. Parameters controlling the measurement reports from a UE, such as how often the UE reports and what types of measurements to report, are also configured by the serving node.
When a UE moves at a high speed, the regular handover procedure often does not work well, due to incorrect estimate of the speed of the UE, degraded channel conditions experienced by the UE, and insufficient time for the UE to perform and report measurement results.
One existing method for measuring the speed of a UE is to count the number of handovers the UE has experienced in a given time period. Such method is based on the assumption that base stations or eNBs (evolved NodeBs) are distributed evenly, an assumption that may not always hold true. Another problem with such method is that the UE may experience a sudden change of speed, for example, when entering a highway. In such case, the speed of the UE estimated based on its handover history would not accurately reflect the new speed of the UE. As a result, the regular handover procedure designed for a UE moving at a normal speed may fail to work properly for a UE moving at a high speed. Calls may drop as a result.
A high speed UE also experiences rapidly degrading channel conditions because of the strong interference from neighboring cells, especially the cell the UE is moving towards (i.e., the target cell). Because of the high speed, the UE may have insufficient time to report measurement results, send a handover request, and decode a received handover command before the UE moves out of the current serving cell. Because of the degraded channel conditions, the high-speed UE may fail to receive the handover command from the serving cell.
There is a need for improving regular handover procedures to accommodate high-speed UEs. The present application discloses robust handover methods and apparatus that reduce call drops and handover failures during handover of high speed UEs.